Tool and method for bonding layers of a metallic axisymmetric structure having complex curvatures

ABSTRACT

A tool and method for bonding layers of a shell by the differential pressure bonding process. The tool and method includes a plurality of separable mandrel segments that combine to form a mandrel having a longitudinal axis, an outer surface, an upper end, and at least one substantially continuous inner surface. The inner surface has a substantially axisymmetric shape having complex curvature. In this embodiment, the tool further includes a retort configured to at least partially shroud the outer surface and upper end of the hollow body. The retort includes at least one vacuum port. The tool is configured to facilitate the compression of a plurality of layers of a multi-layer shell having complex curvature as the shell layers and an interdisposed bonding material are heated to an elevated bonding temperature.

CROSS REFERENCE TO RELATED APPLICATION

This present application is a division of, and claims priority to, U.S.Ser. No. 14/060,305 entitled “TOOL AND METHOD FOR BONDING LAYERS OF AMETALLIC AXISYMMETRIC STRUCTURE HAVING COMPLEX CURVATURES,” and filed onOct. 22, 2013. The '305 application is a division of, and claimspriority to and the benefit of U.S. Pat. No. 8,584,726 issued Nov. 19,2013 (aka U.S. Ser. No. 12/029,179 filed on Feb. 11, 2008) entitled“TOOL AND METHOD FOR BONDING LAYERS OF A METALLIC AXISYMMETRIC STRUCTUREHAVING COMPLEX CURVATURES.” All the aforementioned applications areincorporated by reference herein in their entirety for all purposes.

FIELD

The invention generally relates to tools and equipment for bonding facesheets to a honeycomb core to form a composite structure. Moreparticularly, the invention relates to a tool for use in bondingmetallic face sheets to a metallic honeycomb core by the differentialpressure bonding process to form a hollow axisymmetric shell structurehaving complex curvatures, and a method of using such a tool.

BACKGROUND

The differential pressure bonding process can be used to bond togetherlayered metallic materials to produce high-temperature panels and shellsfor use in modern aircraft. Such panels and shells can include an opencell or “honeycomb” metallic core and metallic face sheets covering theopposed faces of the core. The differential pressure bonding processgenerally includes applying a pressure differential across an assemblyof metallic layers to simultaneously compress and bond the materialstogether at an elevated temperature. A bonding material disposed betweenthe metallic layers bonds the sheets together such as by liquidinterface diffusion bonding or brazing. The metallic layers can includetitanium or Inconel® face sheets, titanium or Inconel® honeycomb cores,and the like.

The differential pressure bonding process was developed to provide amethod of compressing an assembly of layered materials together as thematerials are heated to a bonding temperature. The term “differentialpressure” refers to a difference in pressures across a plurality oflayered materials that acts to press the layered materials against aforming surface as the materials are heated and bonded together. Theforming surface can be a surface of a plate, a mandrel, a die, or othertool having a surface profile corresponding to a desired shape of thebonded structure. A method of bonding metallic panels by thedifferential pressure bonding process is described in U.S. Pat. No.5,199,631, assigned to Rohr, Inc., for example.

To produce a metallic shell structure having a simple cylindrical orconical shape using the differential pressure bonding process, metalliccomponents to be bonded (such as a titanium or Inconel® honeycomb coreand titanium or Inconel® face sheets) can be positioned within aone-piece mandrel having a cylindrical or conical inner surface thatcorresponds to a desired outer shape of the cylindrical or conicalshell. A thin layer of a suitable bonding material is applied betweenthe surfaces of the materials to be bonded. As the assembled layers ofmaterials and bonding material are heated to the bonding material'sbonding temperature, a pressure differential is applied across thelayered materials such that the layers are forced against the contouredinner surface of the mandrel. When the layered materials and bondingmaterial are heated to the bonding temperature, the bonding materialmelts and fuses the layered materials together. Because the layeredmaterials have been forced against the contoured inner surface of themandrel by the applied pressure differential during heating of thebonding material, the outer surface of the resultant shell has a shapecorresponding to the mandrel's cylindrical or conical inner surface.Once the shell has cooled, the simple cylindrical or conical shape ofthe one-piece mandrel permits the bonded shell and mandrel to beseparated by longitudinally separating the shell and the mandrel.

Though the differential pressure bonding process described above can beused to produce shell structures having relatively simple cylindrical,conical, or substantially conical shapes, producing high temperatureaxisymmetric shells having complex curvatures by such a process presentsspecial challenges. As used herein, the term “complex curvature” meanshaving concave and/or convex curvatures wherein at least oneintermediate diameter of the shell is either larger or smaller than bothdiameters at the ends of the shell. As described above, after bondinglayers of a cylindrical or conical shell, the shape of the mandrel andthe shell permits the shell and mated mandrel to be separated by pullingthe bonded shell and mandrel apart in a longitudinal direction. Forshells having non-cylindrical and non-conical shapes and substantialconvex and or concave curvatures, however, the bonded shell is entrappedwithin the mandrel such that the substantially rigid shell cannotphysically be extracted from the mandrel in a single piece. One solutionto this problem is to produce the shell in a plurality of generallyconical or generally cylindrical sections using the process describedabove, and joining the formed sections together end-to-end usingconnecting hardware and/or by welding to form a complete shell. Such amulti-section shell necessarily includes at least one girth seam aroundthe shell's circumference.

High-temperature axisymmetric shells like those described above can beused to form portions of modern aircraft engines. For example, such ahigh-temperature metallic shell can form at least a portion of anengine's exhaust nozzle center plug. FIG. 1 shows one example of atypical engine exhaust nozzle center plug 10. The center plug 100includes a centerbody shell 102 and a tail cone 104. The centerbodyshell 102 is joined to the tail cone 14 along a circumferential girthseam 106. The centerbody shell 102 has an axisymmetric convex curvaturewith a maximum diameter corresponding to highlight 108 (indicated by adashed line in FIG. 1). As discussed above, existing methods of bondinga metallic shell having such complex curvatures using the differentialpressure bonding process dictate that the centerbody shell 102 must beconstructed in at least two sections separated at a girth seam 103corresponding to the shell's maximum diameter. Thus the centerbody 102must be constructed of a forward shell portion 112A having a firstgenerally conical shape, and an aft shell portion 112B having a secondgenerally conical shape. The generally conical shapes of the forward andaft shell portions 112A, 112B permits the shell portions to belongitudinally separated from a one-piece mandrel having a correspondingsubstantially conical shape after bonding.

One goal in the design of some modern aircraft engines is to minimizethe noise emitted by the engines, especially during approach andtake-off conditions. Accordingly, modern aircraft engines can includenoise-attenuating panels and noise-attenuating shells designed to atleast partially dissipate noise generated by an engine's combustor andrapidly rotating fan and rotor blades. As shown in FIG. 2, an engine'snoise emissions can be at least partially reduced by providing an outerskin 129 of the center plug's centerbody shell 112 with perforations 120that permit acoustic communication with underlying open cells 122 of theshell's honeycomb core 124. A non-perforated inner skin 127 covers theinside of the open-cell core 124. Such an arrangement is known todissipate acoustic energy via Helmholtz resonance. The depths of theopen cells 122 can be sized to dissipate acoustic energy having atargeted frequency or range of frequencies. In order to maximize astructure's noise-attenuating capability, the percentage of thestructure's exterior surface area associated with a perforated outerskin 129 and underlying open cells 122 should be maximized in order tomaximize the number of active resonant cavities.

As discussed above, existing methods of bonding high-temperature shellshaving complex curvatures using the differential pressure method dictatethat such shells must be constructed in at least two sections, andjoined together end-to-end along at least one circumferential girthseam. Conventional methods of joining separate shell sections along agirth seam can reduce the surface area of a shell that is available foracoustic treatment, because skin perforations and cell cavitiesproximate to the seam often are blocked by connecting hardware and/or bywelds commonly used to join such shell sections. In addition connectinghardware used to join shell sections increases the number of parts, andadds to the overall weight of the structure. Accordingly, there is aneed for a method of bonding metallic layers of an axisymmetric shellhaving complex curvatures by the pressure differential bonding processsuch that shell is produced in a single piece. By producing the shell ina single piece, perforation and cell blockages associated with girthseam connections can be eliminated, and the external surface area of theshell available for acoustic treatment is maximized. In addition,producing a shell in a single piece can reduce the number of parts andthe total weight of the structure, and can reduce overall productiontime.

SUMMARY

In one embodiment, the invention includes a tool for use in bondinglayers of a shell by the differential pressure bonding process. In thisembodiment, the tool includes a plurality of separable mandrel segmentsthat combine to form a mandrel having a longitudinal axis, an outersurface, an upper end, and at least one substantially continuous innersurface. The inner surface has a substantially axisymmetric shape havingcomplex curvature. In this embodiment, the tool further includes aretort configured to at least partially shroud the outer surface andupper end of the hollow body. The retort includes at least one vacuumport.

The invention also includes a segmented mandrel for use in bonding ashell by the differential bonding process. In one embodiment, themandrel includes a substantially axisymmetric lower mandrel segmenthaving a top and a first contoured inner surface, and a substantiallyaxisymmetric upper mandrel segment having a bottom and a secondcontoured inner surface. When the bottom of the upper mandrel segment isconcentrically positioned on the top of the bottom mandrel segment, thefirst and second contoured inner surfaces combine to form asubstantially continuous axisymmetric surface having a complexcurvature.

The invention also includes a method of bonding a plurality of layeredmaterials by differential pressure bonding to produce an axisymmetricshell having an exterior surface with at least one convex or concavecurvature. In one embodiment, the method includes assembling a segmentedmandrel comprising a plurality of separable mandrel segments. Theassembled mandrel has at least one substantially continuous innersurface that substantially corresponds in shape to the exterior surfaceof the shell. In addition, the method includes assembling the layeredmaterials about the inner surface of the assembled mandrel. The methodfurther includes heating the assembled mandrel and the layered materialsto an elevated bonding temperature while establishing a pressuredifferential that presses the layered component materials against theinner surface of the mandrel as the layered materials are bondedtogether.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of typical exhaust nozzle center plug for amodern aircraft engine.

FIG. 2 is a perspective view of a cutaway portion of typicalsingle-degree-of-freedom noise-attenuating panel or shell of a modernaircraft engine structure.

FIG. 3 is a perspective view of one embodiment of a bonding toolassembly for use in bonding a high-temperature axisymmetric shell havingconvex curvatures using the differential pressure bonding process.

FIG. 4 is a partial cross-sectional elevation view of the bonding toolassembly shown in FIG. 3 taken along line 4-4 in FIG. 3, and showing anaxisymmetric shell having complex curvatures assembled within the toolassembly

FIG. 5 is an exploded perspective view of the tool assembly shown inFIGS. 3 and 4.

FIG. 6 is an elevation view of an upper mandrel segment portion of thetool assembly shown in FIGS. 3-5 shown in partial cross-section.

FIG. 7 is an elevation view of a lower mandrel segment portion of thetool assembly shown in FIGS. 3-5 shown in partial cross-section.

FIG. 8 is a perspective view of a lifting ring portion of the bondingtool assembly shown in FIGS. 3-5.

FIG. 9A is a perspective view of a retort portion of the bonding toolassembly shown in FIGS. 3-5.

FIG. 9B is an exploded perspective view of the retort shown in FIG. 9A.

FIG. 10A is a perspective view of a heat shield assembly portion of thebonding tool assembly shown in FIGS. 3-5.

FIG. 10B is an exploded perspective view of the heat shield assemblyshown in FIG. 10A.

FIG. 11 is a flow chart showing a method of bonding components of anaxisymmetric shell having complex curvatures by the differentialpressure bonding process.

DETAILED DESCRIPTION

One embodiment of a bonding tool assembly 10 for use in bonding layeredmetallic materials together to produce an axisymmetric shell havingconvex curvatures using the differential pressure bonding process isshown in FIG. 3. In the shown embodiment, the tool assembly 10 includesa lifting ring 20, a base plate 30, a retort assembly 40, and a heatshield assembly 50. As shown in FIG. 3, the lifting ring 20 can includea plurality of spaced lifting lugs 25 to facilitate lifting and movingthe tool assembly 10 using known lifting methods. The retort assembly 40can be divided into upper and lower halves 40A, 40B to facilitate partloading and unloading. Optionally, the retort assembly 40 can include aplurality of lifting lugs 49. The heat shield assembly 50 also caninclude a plurality of spaced heat shield lifting lugs 59 for lifting,moving and placing the heat shield assembly 50.

As shown in FIG. 4, the bonding tool assembly 10 further includes alower mandrel segment 70, and an upper mandrel segment 80. In theembodiment of the lower mandrel segment 70 shown in FIGS. 4 and 7, thelower mandrel segment 70 includes at least one contoured inner surface72, a lower ledge 78, an upper lip 76, and an upper step 74. Thecontoured inner surface 72 has a shape corresponding to a lower portionof a shell 200 to be bonded using the tool assembly 10. As shown inFIGS. 4 and 6, the upper mandrel segment 80 includes a contoured innersurface 82, a lower lip 86, and a counterbore 84. The contoured innersurface 82 has a shape corresponding to an upper portion of a shell 200to be bonded using the tool assembly 10. As shown in FIG. 4, the uppermandrel segment 80 mates atop the lower mandrel segment 70 such that thelips 76 and 86 cooperate to concentrically align the mandrel segments70, 80 with each other. As also shown in FIG. 4, the contoured innersurfaces 72, 82 of the aligned mandrel segments 70, 80 combine to definea substantially continuous axisymmetric contoured surface that defines acomplete outer surface contour for a shell 200 to be bonded using thetool 10. At their innermost surfaces 72, 82, the lower and upper mandrelsegments 70, 80 are separated by a seam that substantially coincideswith a highlight 208 of the shell 200 along a circumference having amaximum diameter.

In the embodiment shown in FIG. 4, the lower mandrel segment 70 sitsatop the flat upper surface 32 of the base plate 30, and the base plate30 sits atop the flat upper surface 27 of the lifting ring 20. As shownin FIG. 5, the lifting ring 20, base plate 30, lower mandrel 70 andupper mandrel 80 are substantially concentrically aligned with eachother. As shown in FIG. 4, the cylindrical skin 46 of the retortassembly 40 surrounds the cylindrical outer surfaces of the assembledmandrel segments 70, 80, and the upper skin 48 of the retort assembly 40overhangs and covers the top of the upper mandrel segment 80, thereby atleast partially shrouding the mandrels 70, 80. An impermeable flexiblemembrane 100 is positioned over the inner surface of the shell 200. Theflexible membrane 100 can be constructed of stainless steel, titanium orthe like. An upper edge 101 of the flexible membrane 100 is sealed to anupper interior portion of the retort assembly 40, and a lower edge 103of the flexible membrane 100 is sealed to the base plate 30.

As also shown in FIG. 4, the thermal shield assembly 50 isconcentrically positioned inside the assembled mandrel segments 70, 80,and sits atop the flat upper surface 27 of the lifting ring 27. As shownin FIGS. 4 and 5, the thermal shield assembly 50, retort assembly 40,upper mandrel segment 80, and lower mandrel segment 70 areconcentrically nested together atop the base plate 30 and lifting ring20.

One embodiment of a lifting ring 20 for use with the tool assembly 10 isshown in FIG. 8. In this embodiment, the lifting ring 20 includes a disc22 having an open center. The upper surface 27 of the disc 22 issubstantially flat. A plurality of spaced lifting lugs 25 can beattached to the disc 22 along its periphery.

One embodiment of a retort 40 for use with the tool assembly 10 is shownin FIGS. 9A and 9B. In this embodiment, the retort assembly 40 includesan outer skin 46 and an upper skin 48. The outer and upper skins 46, 48can be constructed of sheet metal, such as type 321 stainless steelhaving a nominal thickness of about 0.06 inch, or the like. Othermaterials also can be used to construct the skins 46, 48. The lowermostedge of the thin outer skin 46 can be reinforced by a stiffener ring 42.The uppermost edge of the outer skin 46 can be connected to the outeredge of the upper skin 48 by an outer angle 43, and the inside edge ofthe upper skin 48 can be reinforced by an inner angle 45. The stiffenerring 42 and outer and inner angles 43, 45 also can be constructed oftype 321 stainless steel, and are welded to the skins 46, 48 bycontinuous gas-tight seal welds all around. As shown in FIG. 4, theinside of the outer skin 46 and at least a portion of the undersurfaceof the upper skin 48 are lined with wire mesh 41. The wire mesh 41 canbe constructed of 304 stainless steel wire having a diameter of about0.063 inch, and can have about seventy-five percent open area. The wiremesh 41 can be tack welded to the inside surfaces of the outer skin 46and the upper skin 48 to secure the wire mesh 41 in place. As shown inFIG. 9A, a plurality of spaced lifting lugs 49 can be welded to theouter angle 43 for use in lifting and placing the retort 40. As shown inFIGS. 9A-9B and 4, at least one vacuum port 60 is provided on the outerskin 46.

If desired, the retort 40 described above can be constructed in upperand lower halves 40A, 40B. As shown in FIG. 4, the retort 40 can besplit along a circumferential seam 47 a that substantially aligns withthe vertical separation between the upper and lower mandrel segments 70,80. As also shown in FIG. 4, the outer skin 46 can be joined and sealedalong the seam 47 a by a membrane 47 b that is welded or otherwisejoined to the inner or outer diameter of the outer skin 46. The membrane47 b can be constructed of a flexible stainless steel material, or thelike.

One embodiment of a thermal shield 50 for use in the tool assembly 10 isshown in FIGS. 10A and 10B. In this embodiment, the thermal shieldincludes a frame 52. The frame 52 can be constructed of welded 304stainless steel pipe having a nominal diameter of about 0.75 inch. Anouter skin 54 surrounds the frame 52, and a concentric inner skin 58lines the interior of the frame. The outer and inner skins 54, 58 can beconstructed of 321 stainless steel having a nominal thickness of about0.06 inch. As shown in FIG. 4, thermal insulation 56 is disposed in theannular region between the inner and outer skins 54, 58. The insulation56 can include one or more layers of Saffil® high-temperatureinsulation, or the like.

One embodiment of a method 300 of using a bonding tool assembly 10 likethat described above to bond an axisymmetric metallic shell havingcomplex curvatures by the differential pressure bonding process issummarized in FIG. 11. In step 301, two or more metallic layers areassembled together to form a “pre-bond” shell assembly 200. For example,the unbonded shell assembly 200 can include an outer skin 129, anopen-cell core 124, and an inner skin 127 like that shown in FIG. 2. Theskins 127, 129 and core 124 can be titanium, Inconel®, or any othermaterial or materials capable of being bonded using the pressuredifferential bonding method. The surfaces of the inner and outer skins127, 129 in contact with the honeycomb core 124 are coated with a thinlayer of a bonding material of a type known in the art. As shown in FIG.4, a lower edge of the shell 200 can be vertically supported by aplurality of spaced riser blocks 90 positioned atop the lower ledge 78of the lower mandrel 70. The riser blocks 90 can be constructed ofgraphite, or the like, and can have a simple rectilinear shape. Theshape of the pre-bond shell assembly 200 substantially conforms to theshape of the inner surfaces 72, 82 of the upper and lower mandrelsegments 70, 80.

In step 303, the lower portion of the pre-bond shell assembly 200 isinstalled into a lower mandrel segment 70 having a first axisymmetriccontoured inner surface 72. In step 305, an upper mandrel segment 80having a second axisymmetric contoured inner surface 82 is installedover an upper portion of the pre-bond shell assembly 200 and onto thelower mandrel assembly 70. As shown in FIG. 4, when the upper and lowermandrel segments 70, 80 are assembled together, they define asubstantially continuous axisymmetric contoured inner surface 72, 82.

In step 307 (and as shown in FIG. 4), a retort assembly 40 is placedaround the mandrel segments 70, 80 and the unbonded shell 200 such thatthe retort shrouds the outside surfaces of the mandrels. A gas tightseal between the lower edge of the retort 40 (e.g. stiffener ring 42)and the base plate 30 can be established by welding a flexible membraneacross the gap between the parts. All other seams of the retort 40 aresealed, preferably by welding. As shown in FIG. 4, a flexible membrane100 is placed over the inner diameter of the unbonded shell 200, andsealed along its top edge 101 to the retort 40, and sealed along itslower edge 103 to the base plate 30.

In step 309 (and as shown in FIG. 4), a thermal shield assembly 50 canbe positioned inside the hollow mandrel segments 70, 80 and the shell200. The thermal shield 50 assembly can be used to shield the shell andmoderate the heating rate of the shell 200, thus substantiallypreventing unwanted thermally-induced deformations of the thin metalliclayers of the shell during heating and bonding.

In step 311, the upper and lower mandrel segments 80, 70 and theunbonded shell 200 are heated to the bonding temperature of the bondingmaterial. As the shell 200 is heated, a pressure gradient is establishedacross the mandrel segments 70, 80 (as described below) such that thelayered materials of the unbonded shell 200 are pressed against thecontoured inner surfaces 72, 82 of the mandrel segments 70, 80.Accordingly, the outer surface of the shell 200 is forced to conform tothe shape of the contoured surfaces 72, 82 as the shell 200 is heated.

Referring to FIG. 4, the required pressure gradient across the mandrelsegments 70, 80 can be achieved as follows. First, a retort assembly 40is placed around the mandrel segments 70, 80 and the unbonded shell 200such that the retort shrouds the outside surfaces of the mandrels. Aflexible membrane can be welded between the lower end of the retort 40and the base plate 30 to form a gas tight seal therebetween. The toolassembly 10 and unbonded shell 200 is placed within a vacuum furnace,and the internal pressure of the furnace is lowered to a pressureP.sub.0 as the furnace is heated. Also as the furnace is heated, avacuum is separately applied to the vacuum port 60 on the retortassembly 40 such that the pressure in the vacuum port 60 and in theoutermost interior regions of the retort 40 surrounding the outercircumferences of the mandrel segments 70, 80 is at or about a pressureP.sub.1. Pressure P.sub.1 should be at least slightly lower than theinternal pressure P.sub.0 such that the pressure differential(P.sub.0-P.sub.1) acts radially outward on the flexible membrane 100 andshell 200. For example, P.sub.1 can be about one psi (pound per squareinch) less than pressure P.sub.0. Both P.sub.1 and P.sub.0 can be nearabout 10.sup.-3 torr to about 10.sup.-5 torr or higher. The resultingpressure differential (.DELTA.P=(P.sub.0−P.sub.1)>0 psi) forces theflexible membrane 100 against the inner diameter of the unbonded shell200, and forces the layered materials of the unbonded shell 200 againstthe contoured inner surfaces 72, 82 of the mandrel segments 70, 80.Accordingly, as the shell 200 and brazing material is heated, the outersurface of the shell 200 is forced to conform to the contoured shape ofthe inner surfaces 72, 82 of the mandrels 70, 80, and the shellmaterials are compressed together.

Returning to FIG. 11, the shell 200 and tool assembly 10 are heated 311until the brazing material melts and diffuses between the compressedcomponent materials of the shell 200. For example, the furnace may beheated to between about 1600 degrees F. and about 2000 degrees F. forabout 1 to 6 hours, depending on the bonding characteristics of theshell and bonding materials. Once the bonding material has melted, theupper and lower mandrel segments 80, 70 and the shell 200 are cooled 313to about room temperature, thereby causing the bonding material tosolidify and to bond the shell components together. Once the shell 200and mandrel segments have cooled, the upper mandrel 80 and retort 40 canbe separated 315 from the shell 200 and from the lower mandrel segment70. For the bonding tool assembly 10 shown in FIG. 4, the retortassembly 40 must be removed before the upper mandrel segment 80 isseparated from the shell 200 and from the lower mandrel segment 70. Inaddition, the thermal shield 50 should be removed before removing theupper mandrel segment 80 in order to improve access to the upper mandrelsegment 80. As can be seen by examining FIG. 4, once the flexiblemembrane 100, retort 40 and upper mandrel segment 80 are removed, thebonded shell 200 can be longitudinally separated 317 from the lowermandrel segment 70 without obstruction.

The embodiments of the invention described above are intended toillustrate various aspects and features of the invention, and are notintended to limit the invention thereto. Persons of ordinary skill inthe art will recognize that various changes and modifications can bemade to the described embodiments without departing from the invention.For example, though the tool assembly 10 has been described as includingtwo mandrel segments 70, 80, a tool assembly according to the inventionmay include more than two separable mandrel segments. In addition,though the mandrel segments have specifically been described as beingseparated along a joint that is transverse to their longitudinal axes,the mandrel segments could be separable along two or more longitudinaljoints. These and all such obvious variations are intended to be withinthe scope of the appended claims.

What is claimed is:
 1. A method of bonding a plurality of layeredmaterials by differential pressure bonding to produce an axisymmetricshell having an exterior surface with at least one convex or concavecurvature, the method comprising: (a) assembling a segmented mandrelcomprising a plurality of separable mandrel segments, the assembledmandrel having at least one substantially continuous inner surface thatsubstantially corresponds in shape to the exterior surface of the shell;(b) assembling the layered materials about the inner surface of theassembled mandrel; and (c) heating the assembled mandrel and the layeredmaterials to an elevated bonding temperature while establishing apressure differential that presses the layered component materialsagainst the inner surface of the mandrel as the layered materials arebonded together.
 2. The method of claim 1, further comprising: (a)cooling the mandrel and the bonded shell; (b) removing at least a firstmandrel segment from the assembled mandrel; and (c) separating the shellfrom the remaining mandrel segment or segments of the mandrel assembly.3. The method of claim 1, further comprising: (a) covering a substantialportion of the assembled segmented mandrel with a retort before heating,the retort having at least one vacuum port; (b) drawing a vacuum throughthe vacuum port to establish the pressure differential.
 4. The method ofclaim 1, further comprising thermally shielding an inside surface of theshell during heating.
 5. The method of claim 1, wherein the segmentedmandrel comprises a lower mandrel segment and an upper mandrel segmentatop the lower mandrel segment, and wherein the upper and lower mandrelsegments are separable at a joint that substantially coincides with ahighlight of the exterior surface of the shell.